Colour photography



United States Patent @r (IQLOUR PHOTUGRAPHY Gordon H. Burt, Wellington, New Zealand, assignor to Trn Colour Film Limited, Wellington, New Zealand, a company of New Zealand No Drawing. Application September 2, 1952, Serial No. 307,567

Claims priority, application Great Britain August 31, 1951 Claims. (CI. 95-88) This invention relates to colour photography and more particularly to improvements in the production of colour records in the production of which thiourea and functionally similar amides and acid dyestuffs are used.

in prior United States Patent specification No. 2,107,094 there is disclosed a process in which a bleached silver image is treated with an acid dyestulf and with thiourea or other thioamide to form a dye-toned image which is then available for transfer to another base when this is required-ms in the production of a three-colour image. Prior to such transfer the dye-toned image is treated in a transfer bath in order to condition it. Such a bath may be an aqueous solution of potassium bromide containing a mineral acid. The treated image is then brought into intimate contact with a gelatine layer car ried upon a permanent base and maintained in contact for a sufficient time for transfer of dye from the dyetoned image to the layer to take place. The process may be repeated a plurality of times using difierent coloured dye-toned images in order to transfer to a permanent base a plurality of colour records and so produce a multicolour image. The images are then fixed.

In operating this process several difficulties have been encountered. For example, ditficulty has been encountered in securing colour records consistently in which the coloration is proportional to the amount of silver present in the developed black and white positive record. So far it has not been possible satisfactorily consistently to overcome this or several other difliculties, which have been observed.

It is accordingly an object of the present invention to provide an improved process for producing colour records which is free from the difiiculties inherent in the known process.

According to the present invcntionthere is provided a process for producing a positive colour record in which a latent silver image is developed in a black and white o'eveloper, the resulting positive image bleached, treated with an aqueous solution of a water-soluble thioamide or selenoamide and with an at least sparingly watergsoluble dyestutf, in which the treatment with thioamide or selenoamide and dyestulf is carried out simultaneously in the presence of a substance which reduces the watersolubility of the silver salt present in the bleached image. According to a feature or" the invention the treatment is carried out at a pH below 7. Desirably the said substance is one which in aqueous solution gives rise to the same anion or anions as the silver saltor salts present in the bleached image.

Experiment indicates that the nature of What occurs when a thioamide, such as thiourea, or a selenoamide, such as selenourca, is brought into contact with a silver salt depends to a great deal upon the circumstances, such as the pH of the medium and the nature of the substances present in the medium. It has been reported that thiourea acts as a solvent for silver halides but under acid conditions (pH below 7) it may not only actas a solvent. When an excess of potassium bromide is added to a silver nitrate solution, followed by gradual addition ofthiourca in aqueous solution, the initial precipitate of silver bromide gradually dissolves but dilution of the resulting solution of silver salt with water does not resultin reprecipitation. The greater the excess of bromide the greater the amount of aqueous thiourea required to effect solution. With about a 10% excess of potassium bromide over that required to react with the silver nitrate approxirnatelyfour mols of thiourea (based on the silver nitrate) are required for complete solution whereas with an approximately excess rather more than seven mols of thiourea are reqiured with the same rate of addition and stirring. Reduction of the pH, for example, by the addition of a small quantity of dilute sulphuric acid, also increases the amount of thiourea required. When the pH is reduced by addition of sodium dihydrogenphosphate the precipitated silver halide goes into solution more readily thanin the presence of sulphuric acid but, on dilution with water, reprecipitation occurs.

The addition of ammonium and sodium bromides has been found to have the same effect as potassium bromide. When other salts are present in place of any one of the bromides the amount of aqueous thiourea required to dissolve freshly precipitated silver bromide varies quite widely being much less with sodium acetate than with sodium sulphate. This difference in behaviour finds a parallel in the behaviour or positive bleached imageswhen they are treated with thiourea and a dyestuif in the presence of added salts. When a bromide is the sole added salt, or is present together with one or more other salts, such as sodium acetate, the dyestutf is taken up in quantities proportional to local densities.

it will be appreciated that to get satisfactory colour values in finished multi-color prints the colour values present on the temporary base-must be produced in linear relation tothe amount of silver present and the transfer to the permanent base must either be substantially complete or linearly proportionate to the colour values present.

The effects produced are clearly related to the nature of the product or products formed between the thioamide, the silver halide or halides present and the dyestuff and the influence exerted by other substances present upon the nature of those products. The presence of an alkali metal or ammonium bromide in the aqueous mixture of thioamide and dyestutf will depress the solubility of silver bromide and this appears to favour the formation of a type of silver halide-thioamide-dyestuff complex in linear or substantially linear proportion to the amount of silver present in the bleached image and such complex is evidently in such form that the dyestutf can be transferred in a form which becomes substantive in contact with the mordant-carrying permanent base. This type of complex appears to be an insoluble one which can be Washed without substantial decomposition. By proceeding in this Way greatly improved results havebeen consistently obtained. Moreover with but small amounts of silver available for complex formation at any given time the silver has little chance to diffuse and the complex is produced inthe correct position.

The formation of such a complex is indicated by the fact that when silver bromide is produced by double decomposition with an alkali metal or ammonium bromide and the resulting precipitate has been dissolved in aqueous thioamide the addition of an aqueous solution of an acid dyestuit results in the formation of a coloured. precipitate. Under these conditions there does not appear to be formation of a mordant image as has been postulated by other workers when working with thiourea under-other conditions. It appears to be essential to the "present process that a coloured complex be formed in linear relation to the amount of silver present, which is substantially insoluble under the reaction conditions. Moreover since the solubility of the silver salt is reduced under the re action conditions full penetration of the complex-forming reagents to the whole depth of the silver halide image is more readily possible and the risk of the formation in the surface layers of the image of a complex which may effectively bar further penetration of the applied reagents into the image is substantially or wholly obviated. This will clearly enable formation -of the desired complex in depth in amounts in or approximating to linear proportionality with the amounts of silver halide present. Thus the present process uses a complex as a source from which to produce a mordant image which appears only on the permanent base and the conditions of formation of the substantially insoluble complex are such as to substantially eliminate undesirable side reactions.

The formation of a complex in the manner indicated will take place within the colloid layer substantially where the silver salt is located and hence the formation of the final mordant image involves migration of dyestuff from within that layer on to the final mordanted base.

The amide employed in the process may be thiourea, selenourea or a substitution product of either of these, such as allyl thiourea, or thioacetamide.

As already indicated, the preferred materials for reducing the solubility of the silver salt are the alkali metal and ammonium halides corresponding to the halide or halides present in the bleached silver image, when the bleached image contains silver bromide.

Any acid dyestuff which will provide an image of the desired hue and form an insoluble complex under the reaction conditions may be employed. The dyestuffs may, and in general will, be selected from different chemical groups, for example, azo, pyrazalone and anthraquinone dyestuffs and will have one or more groups which impart appreciable water-solubility, e. g. carboxyl or sulphonic groups. Examples of suitable dyes for use in three-colour work are Kiton Red 613 (Schultz No. 110), Acid Fuchsin (Schultz No. 800), Lissamine Red 6B (Schultz No. 110), Kiton Yellow S (Schultz No. 732), Tartrazine (Schultz No. 737) and Alizarin Sapphire Blue CB (Schultz No. 1187). The following is an example of a complex-forming bath incorporating one of these dyestuffs:

Kiton Red 6B gm 1 Thiourea gm 0.1 Potassium bromide gm 0.1 Acetic acid 1% aq. soln ccs 2 Water to make 1000 ccs.

According to a feature of the invention the bleached silver image is produced by treating a silver image which has been developed with a black and white developer with a bath containing hydrochloric acid and an oxidising agent, preferably potassium permanganate and then converting the thus formed silver chloride to silver bromide in a second bath which preferably contains sodium bisulphite or potassium metabisulphite and an alkali metal bromide, preferably potassium bromide. The following are examples of these two baths:

(I) Potassium permanganate gms 2 Hydrochloric acid (pure) ccs 36 Water to make 2000 ccs.

(II) Sodium bisulphite gms 10 Potassium bromide gms 10 Water to make 2000 cc-s.

The full procedure for the production of a colour record may be as follows. An image is printed and developed in the conventional manner in a black and white developer. It is then bleached in the first bleach bath for 3-5 minutes, washed thoroughly and then treated inthe second bath for 5 minutes, and again washed thoroughly. Until ready for complex formation the sheet is maintained in a solution of 5 ccs of 0.1% acetic acid made up to 1 litre. To form a blue image complex the bleached sheet is immersed in the following bath:

Alizarin Sapphire Blue CB 150% gm 1 Potassium Bromide 10% aq. soln ccs Sodium dihydrogen phosphate 10% aq. soln ccs 20 Thiourea 1% aq. soln ccs 40 Distilled water to make 24-50 ccs.

or if a magenta image complex is desired the following bath is used:

Lissamine Red 6B 200% gm 1 Potassium Bromide 1% aq. soln ccs 20 Sodium dihydrogen phosphate 10% aq. s0ln ccs 10 Thiourea 1% aq. soln ccs 2O Distilled water to make 2350 ccs.

After five to ten minutes formation of the complex is usually completed. The treated image-bearing sheet is then washed in changes of still water or in gentle flowing water until the non-image bearing portions are quite clear of any colour. The sheet bearing the image complex is then ready for the transfer bath in which it is treated with a solution containing 2.5 gms. of thiourea and 0.5 cc. 99% glacial acetic acid per litre. The sheet is left in this bath for about 10 minutes and is then ready for transfer to a mordanted base material, for example, a gelatine-coated paper which has been mordanted with aluminium hydroxide. To effect the transfer the sheet and the mordanted paper are brought into face-to-face contact and maintained in contact for 5 to 15 minutes at about F. or until transfer is substantially complete.

Any substance which may conveniently be used in a photographic bath for the maintenance of a pH below 7 may be present in the complex-forming bath when, for example, dyestuffs having carboxylic and/or sulphonic acid groups in the molecule are used, but it is preferred to use materials by means of which a pH between 4 and 6.5 can be readily maintained, such as an alkali metal dihydrogen phosphate or an alkali metal buffer or boric acid.

I claim:

1. In a photographic process for producing a positive colour image comprising forming a substantially insoluble silver bromide-amide-acid dyestuff complex by treating silver bromide containing bleached silver image with an aqueous bath containing a substance selected from the group consisting of water-soluble thioamides and selenoamides and with an aqueous bath containing an acid dyestuif and thereafter transferring the dyestuff component of said complex to a permanent base, the improvement in the step of forming said complex which comprises treating said bleached image with an aqueous bath containing (a) a low concentration of said amide (b) an acid dyestuff and (c) a water-soluble bromide salt, whereby said complex is formed in amounts in substantial linear proportionality to the silver present in said bleached image.

2. In a photographic process for producing a positive colour image comprising forming a substantially insoluble silver bromide-thioamide-acid dyestuif complex by treating a silver bromide containing bleached silver image with an aqueous bath containing a substance selected from the group consisting of water-soluble thioamides and selenoamides and with an aqueous bath containing an acid dyestuif and thereafter transferring the dyestuif component of said complex to a permanent base, the improvement in the step of forming said complex which comprises treating said bleached silver image with an aqueous bath containing (a) a low concentration of said amide (b) an acid dyestuff and (c) a water-soluble bromide salt whereby said complex is formed in amounts in substantial linear proportionality to the silver present in said bleached image.

3 The process of claim 2 in which said water-soluble bromide salt is an alkali metal bromide.

4. The process of claim 2 in which said water-soluble bromide salt is potassium bromide.

5. The process of claim 2 in which said water-soluble bromide salt is ammonium bromide.

6. In a photographic process for producing a positive colour image comprising forming a substantially insoluble silver bromide-thioamide-acid dyestuff complex by treating a silver bromide containing bleached silver image with an aqueous bath containing a substance selected from the group consisting of water-soluble thioamides and selenoamides and with an aqueous bath containing an acid dyestuff and thereafter transferring the dyestuff component of said complex to a permanent base, the improvement in the step of forming said complex which comprises treating said bleached silver image with an aqueous bath having a pH less than 7 and containing 6 (a) a low concentration of said amide (b) an acid dyestuff and (c) a water-soluble bromide salt whereby said complex is formed in amounts in substantial linear proportionality to the silver present in said bleached image.

7. The process of claim 6 in which the pH of said bath is between 4 and 6.5 and said water-soluble bromide salt is an alkali metal bromide.

8. The process of claim 7 in which said amide is thiourea.

9. The process of claim 6 in which the pH of said bath is between 4 and 6.5 and said water-soluble bromide salt is ammonium bromide.

10. The process of claim 9 in which said amide is thiourea.

References Cited in the file of this patent UNITED STATES PATENTS 2,107,094 Townsend et al Feb. 1, 1938 

1. IN A PHOTOGRAPHIC PROCESS FOR PRODUCING A POSITIVE COLOR IMAGE COMPRISING FORMING A SUBSTANTIALLY INSOLUBLE SILVER BROMIDE-AMIDE-ACID DYESTUFF COMPLEX BY TREATING SILVER BROMIDE CONTAINING BLEACHED SILVER IMAGE WITH AN AQUEOUS BATH CONTAINING A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF WATER-SOLUBLE THIOAMIDES AND SELENOAMIDES AND WITH AN AQUEOUS BATH CONTAINING AN ACID DYESTUFF AND THERAFTER TRANSFERRING THE DYESTUFF COMPONENT OF SAID COMPLEX TO A PERMANENT BASE, THE IMPROVEMENT IN THE STEP OF FORMING SAID COMPLEX WHICH COMPRISES TREATING SAID BLEACHED IMAGE WITH AN AQUEOUS BATH CONTAINING (A) A LOW CONCENTRATION OF SAID AMIDE (B) AN ACID DYESTUFF AND (C) A WATER-SOLUBLE BROMIDE SALT, WHEREBY SAID COMPLEX IS FORMED IN AMOUNTS IN SUBSTANTIAL LINEAR PROPORTIONALITY TO THE SILVER PRESENT IN SAID BLEACHED IMAGE. 